The attitude control system architecture of large low-earth orbit satellites typically includes an independent back-up safe-hold mode. Such low-earth orbit satellite systems are typically not momentum-biased and therefore must be actively controlled about all three axes. These satellites employ gyros to measure spacecraft attitude and thrusters and/or reaction wheels for attitude control in their safe-hold mode. Upon detection of an attitude error, typically the low-earth orbit satellite is controlled to steer its solar wings toward the sun.
Some geosynchronous orbit momentum biased satellites have a back-up mode which controls a momentum wheel to a fixed rate upon detection of an attitude error with the result that the entire spacecraft, including the solar wings, rotates. However, the spacecraft's health can be guaranteed in this mode for only a few hours because full solar power is not available. On some spacecraft, where the solar wings are automatically rotated ("clocked") at the orbit rate during normal operation, the solar wings may continue to rotate at the fixed rate after detection of a spacecraft failure. However, even if the momentum wheel is controlled to a fixed rate, without closed-loop control of the spacecraft or solar wings using an inertial or optical sensor, the solar wings will drift away from a sun pointing attitude within a few minutes. To ensure long-term stability, the spacecraft must be nutationally stable in the safe-hold mode. That is, the spacecraft must be configured so that nutation does not tend to grow. Although some spacecraft, including geosynchronous orbit momentum biased satellites, have used sun sensors on the solar wings as part of a closed-loop control system to point the wings to the sun, sun sensors mounted on solar wings have not been used as part of a fault-protection system.
Thus a method is required to place a momentum-biased spacecraft in a nutationally stable momentum state and increase the period over which spacecraft health can be guaranteed. Also a method is required to control the spacecraft attitude to direct the solar wings toward the sun.